Sema: Diagnose captures of dynamic 'Self' type from default argument expressions

We could actually allow this for local functions, but it's not
worth implementing that until the more general ability of local
function default arguments to capture values is implemented.

Fixes <rdar://problem/55119566>.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -2655,6 +2655,8 @@ ERROR(dynamic_self_invalid_method,none,
       "covariant 'Self' can only appear at the top level of method result type", ())
 ERROR(dynamic_self_stored_property_init,none,
       "covariant 'Self' type cannot be referenced from a stored property initializer", ())
+ERROR(dynamic_self_default_arg,none,
+      "covariant 'Self' type cannot be referenced from a default argument expression", ())
 
 //------------------------------------------------------------------------------
 // MARK: Type Check Attributes

lib/Sema/TypeCheckCaptures.cpp

@@ -624,6 +624,11 @@ void TypeChecker::computeCaptures(AnyFunctionRef AFR) {
                                 /*isObjC=*/false);
         E->walk(finder);
 
+        if (finder.getDynamicSelfCaptureLoc().isValid()) {
+          Context.Diags.diagnose(finder.getDynamicSelfCaptureLoc(),
+                                 diag::dynamic_self_default_arg);
+        }
+
         auto captures = finder.getCaptureInfo();
         if (isGeneric)
           captures.setGenericParamCaptures(true);

test/decl/func/dynamic_self.swift

@@ -437,3 +437,11 @@ class Iterable : Sequence {
     return DummyIterator()
   }
 }
+
+// Default arguments of methods cannot capture 'Self' or 'self'
+class MathClass {
+  func invalidDefaultArg(s: Int = Self.intMethod()) {}
+  // expected-error@-1 {{covariant 'Self' type cannot be referenced from a default argument expression}}
+
+  static func intMethod() -> Int { return 0 }
+}